An Organic Light-Emitting Diode (OLED) display, which is also called as an organic electroluminescent display, is an emerging panel display device. It has a wide application prospect because it has various advantages, such as simple preparation processes, low cost, low power consumption, high brightness of light emitting, wide adaption scope of operation temperature, light and thin cubage, rapid response speed, being apt to realize a color display and a large screen display, being apt to be matched with a Gate on Array, being apt to realize a flexible display, and the like.
The OLED pixel units in the organic electroluminescent display are generally arranged in a matrix, and may be classified into two driving modes of a Passive Matrix-Organic Light Emission Display (referred to as a PM-OLED briefly) driving mode and an Active Matrix-Organic Light Emission Display (referred to as an AM-OLED briefly) driving mode, according to their different driving modes in the driving circuit for the OLED pixel unit. Herein, the PM-OLED driving mode fails to satisfy requirements for the display with a high resolution and a large size because it has disadvantages of crosstalk, high power consumption, short lifespan, etc., although its process is simple and cost is low. As compared, the AM-OLED driving mode integrates a set of thin film transistors (referred to as TFTs briefly) and a storage capacitor in each pixel unit, in order to compose the pixel unit pixel unit driving circuit, and realizes controlling of a current passing through the OLED by a driving control of the TFT, so that the OLED emits light. Because of the addition of the TFTs and the storage capacitor, the OLED in the pixel unit driving circuit may always emit light within a controllable period of one frame, and the required driving current is small, the power consumption is low and the lifespan is longer, which may satisfy the requirements for the large size display with the high resolution and multiple grey scales. Meanwhile, the AM-OLED has obvious advantages in many aspects like angle of view, restoring of colors, the power consumption, the response time and the like, and is applicable to the display device with high information amount and high resolution.
As illustrated in FIG. 1, the exiting AM-OLED pixel unit driving circuit utilizes generally a structure of 2T1C, that is, it comprises two thin film transistors and one storage capacitor, which are a switch transistor T1, a driving transistor T2 and the storage capacitor Cs, respectively. The driving mode thereof may comprise two stages, namely, a data writing stage and a data retaining stage.
During the data writing stage, a scan line of the AM-OLED driving circuit outputs a row selection signal VSel to select a row at which the switch transistor T1 locates, so that the switch transistor T1 is turned on, a data voltage Vdata outputted from a data line of the selected row enters into the pixel unit via the switch transistor T1 and charges the storage capacitor Cs. With the gradually increasing of potential at a gate of the driving transistor T2, the driving transistor T2 starts to be turned on. During a stable programming stage, the driving transistor T2 operates in a saturation zone, and the output current (that is, a current passing through the OLED) of the driving transistor T2 is as follows, according to a source-drain current formula for the saturation zone of the TFT:
                              I                      T            2                          =                              I            OLED                    =                                    1              2                        ⁢                          μ              n                        ⁢                          C              ox                        ⁢                          W              L                        ⁢                                                            (                                                            V                      GS                                        -                                          V                      th                                                        )                                2                            .                                                          (        1        )            
In the Equation (1), μn is an electron mobility, Cox is the capacitance at the insulation side of the OLED per unit area, W is a channel width of the driving transistor T2, L is a channel length of the driving transistor T2, VGS is a gate-source voltage of the driving transistor T2, and Vth is a threshold voltage of the driving transistor T2.
During the data retaining stage, the row select signal VSel outputted from the scan line of the AM-OLED driving circuit does not select the row at which the switch transistor T1 locates, so that the switch transistor T1 is turned off. At this time, the potential at the gate of the driving transistor T2 is kept unchanged because of an effect of charges stored in the storage capacitor Cs, and thus the driving transistor T2 is kept to be in a turn-on state. Meanwhile, the OLED emits light to realize gray scales under a control of a given power supply voltage Vdd, and the OLED is kept to be driven continuously during the data retaining stage.
It can be seen from the current expression (namely the Equation (1)) of the OLED, this current not only is controlled by the data voltage Vdata, but also is affected by the threshold voltage Vth of the driving transistor T2, that is to say, the existing 2T1C structure fails to compensate for a shift or an inconsistency of the threshold voltage Vth. Because it is impossible for the driving transistors T2 in the respective pixel unit driving circuits to have completely consistent performance parameters, and the threshold voltage Vth of the driving transistor T2 in the respective pixel unit driving circuits are not compensated for, the currents flowing through the OLEDs in the respective pixel units would be inconsistent, such that the brightness of the light emitted from the respective pixel units are non-uniform, which leads to a non-uniformity in the brightness of the whole display screen and in turn affects a display effect. Further, because the currents flowing through the OLEDs in the respective pixel units have a non-linear relationship with the data voltage Vdata, it is not benefit for adjusting the gray scales in the whole display screen.